Automatic weight paint error when trying to rig a bow
This warning usually appears when there is a bone that doesn't have anything near it. 

The bone that is supposed to pull the string is the likely culprit.   

I suggest applying with envelope weights, (which I was able to do after ridding myself of the pesky draw bone) and getting on your thinking shoes for some intensive weight painting time. Sometimes you just need to weight paint because the computer's automatic isn't 'human' enough. This can take forever, so don't do it before a mid-day appointment.

Also, your string is just kinda stuck into the bow; you might want to separate it from the bows shaft and then join them using a boolean modifier set to union.